1. Field of the Invention
This invention is directed to a novel solid electrolytes derived by the polymerization of a diacrylate monomer having a recurring rigid (i.e., hard) alkane segment (as described).
This invention is further directed to a solid electrolytic cell (battery) containing an anode, a cathode and a single phase solid electrolyte comprising a solid polymeric matrix containing recurring units derived from the specific diacrylate monomer, an inorganic ion salt, and a solvent.
2. State of the Art
Electrolytic cells containing an anode, a cathode and a solid, solvent-containing electrolyte are known in the art and are usually referred to as "solid batteries". These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., "liquid batteries") including improved safety features.
The solid, solvent-containing electrolyte employed in such solid batteries has heretofore contained either an inorganic matrix or an organic polymeric matrix as well as a suitable inorganic ion salt. Because of their expense and difficulty in forming into a variety of shapes, inorganic non-polymeric matrices are, however, not preferred and the art typically has employed a solid electrolyte containing an organic or inorganic polymeric matrix.
Suitable organic polymeric matrices are well known in the art and are typically organic homopolymers obtained by the polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283 or copolymers obtained by polymerization of a mixture of organic monomers. Suitable organic monomers include, by way of example, polyethylene oxide, polypropylene oxide, polyethyleneimine, polyepichlorohydrin, polyethylene succinate, and an acryloyl-derivatized polyalkylene oxide containing an acryloyl group of the formula CH.sub.2 =CR'C(O)O-- where R' is hydrogen or lower alkyl having from 1 to 6 carbon atoms.
Additionally, suitable organic monomers preferably contain at least one heteroatom capable of forming donor acceptor bonds with inorganic cations (e.g., alkali ions). When polymerized, these compounds form a polymer suitable for use as an ionically conductive matrix in a solid electrolyte.
The solid electrolytes also contain a solvent (plasticizer) which is added to the matrix primarily in order to enhance the solubility of the inorganic ion salt in the solid electrolyte and thereby to increase the conductivity of the electrolytic cell. In this regard, the solvent requirements of the solvent used in the solid electrolyte have been art recognized to be different from the solvent requirements in liquid electrolytes. For example, solid electrolytes require a lower solvent volatility as compared to the solvent volatilities permitted in liquid electrolytes.
Suitable solvents well known in the art for use in such solid electrolytes include, by way of example, propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, triglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane, and the like.
Heretofore, the solid, solvent-containing electrolyte has typically been formed by one of two methods. In one method, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of the inorganic ion salt and the electrolyte solvent (a glyme and the organic carbonate) are then added to the solution. This solution is then placed on the surface of a suitable substrate (e.g., the surface of a cathode) and the volatile solvent is removed to provide for the solid electrolyte.
In the other method, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the inorganic ion salt and the solvent. This mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (e.g., heat, ultraviolet radiation, electron beams, etc.) so as to form the solid, solvent-containing electrolyte.
While the electrolytes described above perform adequately in their intended role, there is need for improvement in several areas. First, the conductivity of the electrolyte could advantageously be increased. Cumulative capacity of a solid battery is the summation of the capacity of the battery over each cycle (charge and discharge) in a specific cycle life.
Second, the electrolyte must be compatible with the typically used inorganic ion salt incorporated into the polymer matrix to aid in conductivity. The inorganic ion salt, which usually contains Li ion but which can contain other metal ions as discussed hereinafter, must be soluble in the electrolyte to form a one-phase system. Hence the amount of salt which can be incorporated in the electrolyte is limited by the salt's saturation concentration. By providing an electrolytic environment in which the salt is more soluble, more of the salt can be incorporated and hence conductivity can be increased.
Third, the solid polymeric matrix must have a certain degree of flexibility and swellability in order to function properly in the cell. If the cross-linking density is too high, the resulting polymer network is very tight, resulting in minimal flexibility and swellability, and possibly a very brittle material.